Coking process and apparatus



Nov. 5, 1968 B. v. MOLSTEDT 3,409,542

COKING PROCESS AND APPARATUS Filed Dec. 21, 1966 -BURNER 15-- 7 +FLUEGAS ""CYCLONE 8-v- REACTOR W GAS GAS FLUIDIZING GAS B. Inventor UnitedStates Patent 3,409,542 COKING PROCESS AND APPARATUS Byron V. Molstedt,Baton Rouge, La., assignor to Esso Research and Engineering Company, acorporation of Delaware Filed Dec. 21, 1966, Ser. No. 603,490 8 Claims.(Cl. 208127) ABSTRACT OF THE DISCLOSURE A process and apparatus aredescribed for coking hydrocarbons wherein heat is supplied to theprocess by upflow transfer of coke particles with liberated gasesthrough a transfer line heater and the heated particles are thenreturned to the coking zone. The top of the reactor is tapered in atrumpet shape. The disclosed invention is especially useful in a hightemperature coking process.

This invention relates to the fluid coking of hydrocarbons, especiallypetroleum hydrocarbons. More particularly, it relates to a process andapparatus for making high temperature fluid coke wherein the endothermicheat for the process is supplied to the coke by burning combustiblegases from the coking process itself.

It is known to prepare coke in a fluidized bed at low temperature, e.g.,850 to 1200 F., and at high tempera tures, e.g., 1800" to 2800 F. Atypical fluid coking process and apparatus is described in US. PatentNo. 2,881,- 130. A high temperature fluid coking unit consists basicallyof a reactor vessel for conducting the coking reactions and an auxiliaryheater-burner vessel for supplying heat for the reactions. The reactorvessel or coker is generally a vertical, elongated vessel with a flat orelliptical top and contains a dense, turbulent, fluidized bed of hotcoke particles maintained at a temperature in a range of about 1800 to2800 F. Feedstock, i.e., hydrocarbon, is injected into the coker andcracked essentially to hydrogen and coke, usually with the formation ofa small amount of soot and uncracked, or partially cracked, hydrocarbonsas by-products. Uniform mixing in the fluid bed results in virtuallyisothermal conditions and effects instantaneous distribution of thefeedstock.

The heat for carrying out the endothermic coking or cracking reaction isusually generated in an auxiliary heater or burner vessel. A fluidstream of coke is withdrawn horizontally or downwardly from thefluidized bed of the reactor via a downcomer and transported as a densephase, fluid solids system to a riser or vertically aligned conduit. Inthe riser the coke solids are conveyed upwardly by injection of largequantities of carrier or lift gas, which converts the dense phase ofcoke to a disperse or dust phase. The disperse phase is mixed withoxygen or oxygen-containing gases, such as air, passed through atransfer line heater and heated by partial combustion of the coke solidstherein or by combustion of an extraneous fuel gas added thereto. Suchfuel gas may be added at the bottom of the riser and serve also as thecarrier or lift gas. Sufficient combustion is carried out to maintainthe coker-burner system in heat balance, i.e., by generating enough heatto balance heat losses resulting from the endothermic cracking reaction,the sensible heat in the exiting solids and gaseous coker products, andthe heat losses through the walls of the process equipment.

Certain problems and inetficiencies are inherent in coking operationscarried out at high temperatures. Large quantities of gaseous products,mainly hydrogen, leave the reactor at high temperatures and carry away agreat deal of sensible heat. This greatly lowers the thermal efficiencyof the process.

The prior art coking processes require a great deal of special apparatusfor transferring solids and handling 3,409,542 Patented Nov. 5, 1968gases. Several transfer line risers, downcomers, and cyclones aregenerally required. Separate gas handling facilities are required forthe reactor and the transfer line heater. Thus, the gas leaving thereactor passes through a cyclone or other solids-gas separating means,and similar equipment is required to handle the exhaust gases from theburner. This added equipment is not only extremely expensive as aninitial investment, but it also requires extensive maintenance. Thecomplexity of the apparatus has made the coking operating sensitive toplugging and coking of various lines. It also presents problems due tothe failure of refractory material at the high temperatures involved inthe numerous lines. In any system where there is a great deal of solidscirculation and transfer, the unit downtime generally goes upsubstantially as the number of elements in the apparatus. This isespecially true in a very high temperature process such as hightemperature coking.

Also because of the coarse material of the fluidized coke in a hightemperature coking process, the design of the system is critical. Thecirculated coke solids are very dense, i.e., about 190+ g./cc. with aparticle size range of about 5 to 5000 microns, more generally about 50to 500 microns, with median particle sizes of about to 250 microns. Suchsolids are diflicult to fluidize in that they tend to form fluid bedswhich are extremely sensitive to slugging and the formation of large gasbubbles.

The prior coking techniques also require the compression and addition oflarge quantities of extraneous heating gas as fuel in the transfer lineburner or heater.

Still other gas facilities are required to provide the numerous bleedgas injection points and metering equipment necessary to the operationof the various risers, downcomers and transfer lines.

Another disadvantage of prior art coking processes is that extensivecleanup to remove entrained soot from eflluent process gases is requiredbefore the gases can be recovered or released into the atmosphere. Thecleanup problem is doubly complicated because of the necessity formaintaining two efiiuent streams of gas, i.e., reducing gases from thereactor and flue gases from the transfer line burner.

One method considered for overcoming or avoiding part of these prior artproblems is to take advantage of the entrainment of coke in theliberated hydrogen and other reactor effluent gases to transport thecoke to a heater or burner located above the reactor. This makes use ofthe phenomenoon that a fluid bed of coke is actually composed of twomore or less distinct phases, namely, a dense phase and a dilute phase.Most of the coke is present as a dense phase with a smaller fractionexisting as a dilute phase above the dense phase bed of coke. The densephase is characterized as a pseudo liquid, having a visible level orupper surface. The dilute phase, on the other hand, is much like a gasphase and consists partly of coke thrown up above the dense phasesurface by the boiling or bubbling action of the gas-fluid system.

The lower portion of the dilute phase contains larger particlescontinuously falling back into the dense phase. Normally, the solidsconcentration decreases with height in the dilute phase and, if theheight is sufficient, the upper portion of the dilute phase will containessentially only particles so small as to be entrained in the upflowinggases, i.e., having free fall velocities less than the ascending gasvelocity.

By decreasing the dilute phase height, the entrainment rate can beincreased until coke solids are being entrained out of the reactor atthe same rate at which coke is being produced, thus eliminating the needfor standpipe withdrawal systems.

In such upflow reactors the fluid solids circulation rate through theburner is the same as the entrainment rate out of the reactor. It iscontrolled as a function of the reactor outage, i.e., the distance fromthe dense phase surface to the top of the reactor. As the outagedecreases, the entrainment and circulation rate increases for any givengas rate. The outage adjusts automatically, therefore, to that requiredfor the imposed circulation rate for the system.

When systems containing fluid beds of relatively small particles,averaging 40 to 100 microns with a size distribution in the range offrom about to 250 microns and having a relatively low particle density,i.e., about 0.8 to 1.5 g./cc., are used in the conventional upflowentrainment type circulation system, little difficulty is encountered.In such systems, the fluidization, entrainment and outage effects aregenerally smooth in performance and the geometric design of the reactorvessel is not particularly critical. Satisfactory operation cansometimes even be achieved with solids outside the foregoing ranges. Forexample, more dense particles can be used if they are very small andconversely larger particles can be used if their density is very low.

However, with solids which are both dense and coarse, such as fluidcoke, the geometry of the upper portion of the reactor is extremelyimportant. At superficial gas velocities preferred for high temperaturefluid coking, i.e., 0.3 to 2.0 ft./sec., the upflow gas velocity is tooslow to provide sufiicient entrainment of solids for good solidscirculation unless extremely low outages are employed, i.e., 0.5 to 2feet. These low outages are found to be impractical or inoperable in theconventional system because beds of coarse, dense solids surge and slugunder such conditions, which leads to extreme pressure fluctuations.This behavior is due to massive impacts of the bed on the top of thevessel coupled with intermittent choking of the reactor outlet withdense slugs of solids.

Thus, it was heretofore considered impractical, if not impossible, tocarry out a high temperature coking process in upflow apparatus whereina burner or heater located above the coker or reactor vessel can employthe combustible reactor effluent gases, both as a fuel to supply heat tothe circulating coke in the burner and as a solids transport medium forcarrying or conveying coke particles from the reactor to the heater.

Such a process is now practical and highly advantageous when practicedin accordance with the present invention.

This invention contemplates a process whereby hydrocarbon is heated andcracked essentially to hydrogen and coke in a coking zone containing adense phase bed of fluid coke particles having a bed height-to-diameterratio ranging from about 0.5 to 3, preferably 0.5 to 1.5, and a dilutephase zone of fluid coke particles above and contiguous to the densephase bed. The liberated hydrogen is then passed upwardly atprogressively increasing superficial gas velocities through the cokingzone to entrain coke particles in the dilute phase zone and convey thehydrogen and entrained coke particles as a disperse phase to a heatingor burning zone. In the heating zone an oxygen-containing gas is addedto the disperse phase to combust at least a portion of the hydrogen,thereby heating the coke particles. Heated coke is then separated fromthe disperse phase and returned to the coking zone to provide heat tocarry out the endothermic cracking reaction therein.

The progressive increase in superficial gas velocity can be achieved bypassing the liberated hydrogen through a specially tapered zone untiladequate transport velocities are reached. It is important to increasethe gas velocity smoothly, but rapidly, to avoid surging and slugging ofsolids in the coking zone outlet.

Maximum benefits are achieved when the superficial gas velocity withinthe dense phase fluid bed of coke ranges from about 0.3 to about 2.0ft./sec., preferably about 0.3 to 1.0 ft./sec. and increases tovelocities in the range of 25 to 100 ft./sec. at the top of the taperedzone.

At such conditions it is desirable to maintain the upper surface of thedense phase fluid bed less than about 5 feet, preferably less than 3feet below the tapered zone. More preferably, the surface of the densephase bed will be within the lower part of the tapered zone wheresuperficial gas velocities range from about 1 to about 5 ft./ sec.Smoothest operation is generally achieved when the superficial gasvelocity at the surface of the bed is about 2 to 3 ft./sec.

In a preferred embodiment of this invention, a portion of the fluid cokeparticles is withdrawn from the dense phase fluid bed of the reactor,and conveyed as a dense or disperse phase to the heating zone through aseparate riser, by-passing the tapered zone. The flow rate by weight ofthe coke particles through the tapered zone is about 0.2 to 10 times,and preferably 0.5 to 5 times, the flow rate of coke particlesby-passing the tapered zone. At these conditions, optimum entrainment ofsolids by the liberated hydrogen to provide the smoothest and mostefficient operation of the tapered zone is achieved and any potentialcoke deposition in the top of the reactor or in the outlet iseliminated. Preferably, the total coke rate through the heating zoneranges from about 20 to times the rate at which coke is formed by thecracking of hydrocarbon feed in the cracking zone.

Suitable apparatus according to this invention requires a reactor with asmoothly. tapered top. The top should be generally trumpet shaped,designed according to the following equation:

where H=the height in the tapered top above the reactor proper,

D =diameter of the tapered top at any height H,

D =diameter of the reactor proper,

K=shape factor.

The value of the shape factor, K, ranges from about 1.5 to 2.5 andpreferably about 1.85 to 2.15.

Reactor tops conforming to the foregoing equation will provide gasvelocity gradients which result in the desired entrainment of fluidsolids without slugging when used in combination with a main reactorbody of sufficient size to provide an overall bed height-to-diameterratio of about 0.5 to 3, preferably 0.5 to 1.5. The main reactor body orreactor proper is that portion of the reactor below the tapered topadapted to contain the main body of fluidized solids. In the case ofnon-circular reactors, the diameters D and D defined above are intendedto include pseudo-diameters, i.e., diameters of circles having the samecross-section as the reactor or tapered top in question.

The prefer-red apparatus includes in combination with the tapered topreactor a by-pass riser adapted to transport coke particles at a rate ofabout 0.1 to 5 times, preferably 0.2 to 2 times the rate at which cokeis passed through the tapered top.

As a practical matter of construction, the smoothly tapered trumpetshape of the reactor top can be approximated by a series of at least 2,and preferably 3 or more, converging cones.

The invention will be better understood by reference to the attacheddrawing, which shows an embodiment wherein the upflow of coke particlesin the reactor is provided by gradual tapering of the top of the vesselinto three successive cones, which causes the gas velocity toprogressively increase as it proceeds upwardly in the vessel, therebyconveying entrained coke particles from the reactor into the transferline heater, from which heated coke particles are returned to thereactor via a cyclone separator. To supplement the solids conveyed tothe trans fer line heater through the tapered reactor top, a dense phaseriser is provided in which the solids transfer is controlled by the rateof aerating the transfer line riser. The tapered top is designed toapproximate a trumpetshape in which the diameter, D at any height Habove the main reactor body, whose diameter is D is determined inaccordance with the equation described hereinabove.

Referring specifically to the drawing, hydrocarbon feed is introducedinto the reactor by line 21 and is cracked to coke and hydrogen, thehydrogen comprising the fluidizing gas which maintains fluidization ofdense phase coke bed 2. Any convenient feed can be used, depending onthe cracking temperature employed. Suitable feeds, for example, are lowmolecular weight gases such as methane, heavy atmospheric or vacuumresidua and intermediate naphthas, gas oils and the like. The upflow ofcoke particles and hydrogen product gas proceeds through dilute phasezone 5, through a tapered zone 6, into line 7, through which thedisperse phase of coke and hydrogen is conveyed into transfer lineheater 15. Preheated combustion air is introduced through line 9 intothe transfer line heater and combusted with hot hydrogen therein. Theresulting gases as well as the heated solids, are conveyed downwardlythrough line 16 into cyclone 8 wherein the heated solids are separatedfrom the combustion gases. Off-gases are vented through line 11. Thesolids, at temperatures generally about 200-400 F. above the reactor bedtemperature, are conveyed downwardly through downcomer 10 and areinjected into reactor bed 2 by providing aerating gas through line 19.Additional coke particles are withdrawn from dense bed 2 and conveyedvia riser 12 by means of aerating gas injected through line 20. Cokeparticles circulation is effected by controlling the aerating gas inline 10 and line 12 as well as the bed level in the trumpet-shapedtapered bed reactor. Aerating gases can also be injected into bed 2 vialine 13 to optimize fluidization. Product coke is withdrawn through line14 and about 20 to 40 wt. percent is ground to provide seed particles,which are returned to the fluid bed (by means not shown) to control theaverage particle size within a fluidizable range.

The invention will be better understood by the following example:

Residuum feed is preheated to 500 F. and injected into a coker reactorhaving a height-to-diameter ratio of 1.5. A reactor vessel is usedhaving three successive tapered sections at the top with the includedangles being 60, 30 and 15 for the lower, middle and upper conesrespectively. The tapered sections provide for increasing thesuperficial gas velocity from about 1.0 ft./sec. in the dense phasefluid bed to about 75 ft./sec. at the outlet of the upper cone. Thisincrease in gas velocity provides the lifting means for conveying cokesolids upflow as a disperse phase from the coking reactor to a transferline heater. The level of the dense phase fluid bed is maintained about1 to 2 feet up into the lower cone section where the superficial gasvelocity is about 2 to 3 ft./sec. This causes the gas to transportsolids smoothly out of the tapered zone at the rate of about 05-075pounds of solids per cubic foot of gas. Additional coke solids areconveyed to the transfer line heater through a dense phase riser. Theamount of these solids is controlled at about of the rate at which cokecirculates through the tapered zone of the reactor by controlling therate at which aerating gas is introduced to the dense 'phase riser.

Product coke is withdrawn from this apparatus at a rate sufficient tomaintain the reactor bed level. About /3 of the product coke is groundto a particle size of minus 300 mesh and recycled as seed coke. Thismaintains the average particle size of minus 300 mesh particles in thefluid coke bed between 20 and 30 wt. percent. The coke in the fluid bedhas an average particle size of about 200 microns and an averageparticle density of about 1.9 g'./cc.

Hydrogen from the cracking reaction is evolved at a sufficient rate toprovide a superficial linear gas velocity in the fluid bed of 1.0ft./sec. based on the full reactor diameter below the tapered top. Cokeis withdrawn from the reactor and circulated to the transfer line heaterat a total rate through the dense phase riser and the tapered reactortop of times the production rate of product coke. The hot evolvedhydrogen gas from the reactor is combusted in the burner with airpreheated to a temperature of about 1000 F., to heat the circulatingcoke particles to a temperature of about 2300 F. The heated cokeparticles are separated from the combustion gases in a cyclone separatorand returned via a standpipe to the dense phase bed of coke particles inthe reactor, to maintain the temperature therein at about 1200 F.

The process of the present invention can be carried out with a minimuminvestment cost for apparatus. It also comprises a simple, economicalscheme for carrying out the coking reaction, heating the solids in thetransfer line burner, and providing heat to the coking reaction by thesensible heat of the circulated solids. The apparatus eliminates varioustransfer line risers and downcomers. Most important, no gas-solidsseparator device, such as a cyclone, is required over the reactoroutlet. Moreover, in this process, a self-contained unit is feasible,not only to produce coke, but also to produce its own fuel.

The process can thus be completely independent of the availability ofextraneous hydrocarbon fuels required in conventional prior art cokingprocesses. Provisions can also be made for recovering the excess amountof hydrogen produced by condensing out the combustion water and removingother impurities. High purity coke is obtained because of theelimination of extraneous impurities that may be introduced intoconventional processes which burn an extraneous fuel in the transferline heater.

The invention is not to be limited by the preceding example, which isillustrative. Many other variations of the invention will be apparent tothose skilled in the art.,

What is claimed is:

1. A process for making coke comprising heating and cracking ahydrocarbon feed essentially to hydrogen and coke in a ocking zonecontaining a dense phase bed of fluid coke particles which has aheight-to-diameter ratio ranging from about 0.5 to 3 and a dilute phasezone of fluid coke particles above and contiguous to said dense phasebed,

passing said hydrogen upwardly at progressively increasing superficialgas velocities through said coking zone to entrain coke particles insaid dilute phase zone,

conveying said hydrogen and entrained coke particles as a disperse phaseto a heating zone,

adding oxygen-containing gas to said disperse phase to combust at leasta portion of the hydrogen to heat the coke therein,

separating heated coke from said disperse phase and returning said coketo said coking zone to provide heat to carry out the endothermiccracking reaction therein.

2. The process of claim 1 wherein said superficial gas velocities areprogressively increased by passing said hydrogen upwardly through atapering zone, the cross-section of which decreases with heightapproximately as the cross-section of a trumpet-shaped zone defined bythe equation:

1 DH DR\/K(H/0.25DR) where:

zone at any 3. The process of claim 2 wherein said superficial gasvelocity progressively increases in said tapering zone from a velocityin the range from about 0.3 to 2.0 ft./sec. to a velocity in the rangefrom about 25 to 100 ft./sec.

4. The process of claim 2 wherein the upper surface of said dense phasebed lies less than about 5 feet below said tapering zone.

5. The process of claim 4 wherein said upper surface is within saidtapering zone such that the superficial gas velocity at said surface isin the range from about 1 to 5 ft./sec.

6. The process of claim 2 wherein fluid coke particles are withdrawnfrom said dense phase fluid bed, entrained in a gas, and conveyed as adisperse phase to said heating zone by-passing said tapering zone.

7. The process of claim 6 wherein the ratio of the flow rate by weightof coke particles through said tapering zone to the flow rate of cokeparticles by-passing said zone ranges from about 0.2 to about 10.

8. An apparatus for coking hydrocarbon feed comprising a coking reactoradapted to contain a bed of fluidized coke particles which has aheight-to-diameter ratio in the range from about 0.5 to 3, said reactorhaving a main reactor body and a reactor top which is generallytrumpetshaped and tapered from the diameter of said main reactor body upto a narrow upper zone, said upper zone being in communication with atransfer line heater, said transfer line heater also being in directcommunication with said reactor through a dense phase riser providedwith an aerating gas means, said riser by-passing the tapered reactortop, and means for introducing oxygencontaining combustion gas to saidtransfer line heater, said transfer line heater being in communicationwith a gas-solids separator, and means for removing gaseous combustionproducts from said separator and for passing coke particles back to saidreactor.

References Cited UNITED STATES PATENTS 2,912,315 11/1959 Haney 23288.3

DELBERT E. GANTZ, Primary Examiner.

HERBERT LEVINE, Assistant Examiner.

